Copolymers and terpolymers, when isolated from reaction solution are a mixture of compounds of varying composition and molecular weight. Typically they contain small quantities of starting material and by-products which are undesirable in the final polymer. The polymer mixture is precipitated from the solvent or solvent mixture by adding the mixture to a second solvent, as for example water, hexane, heptane, octane, petroleum ether, or a mixture thereof. The polymer is then dried under a nitrogen atmosphere. The subject of this invention pertains to a method of purifying the polymer.
Davidson, in U.S. Pat. No. 5,945,251, discloses a method of purifying polyhydroxystyrene polymers by adding an amine, a hydrophilic solvent, a hydrophobic solvent, and water to the polymer; separating the aqueous phase; then removing the hydrophilic solvent and the hydrophobic solvent to form the purified polymer.
Zempini, et al. in U.S. Pat. No. 5,789,522 and U.S. Pat. No. 5,939,511, extracts impurities from a phenolic resin by dissolving the resin in a photoresist solvent and extracting the water-soluble impurities therefrom.
The present invention provides a novel process for improving the glass transition temperatures and reducing the polydispersity values of polymer intermediates that have been polymerized by precipitation from methanol. The polymers that are susceptible to treatment with the method of this invention are polymers of 4-acyloxystyrene. The 4-acyloxystyrene derived polymers are then transesterified to 4-hydroxyphenyl-containing polymers useful in paints, resins, thickening agents, and in photoresist compositions. The present invention process is an improvement over the prior art and is quite efficient. Specifically the invention provides a method of removing unreacted monomers, low molecular weight polymers, and the like from the crude polymer mixture before the transesterification step. Many analytical methods can be utilized to quantify the improvement in the purity of polymers. Average molecular weight, nuclear magnetic resonance, chromatography, and glass transition temperature are all effective in certain instances with certain molecules and characteristic side chains.
As previously described in the prior art, the crude polymer after polymerization is separated from the alcohol by filtration, centrifugation, decantation, or the like. According to the method of this invention, the polymer is subject to fractionalization whereby it is suspended in methanol and the solid is separated from the methanol. This procedure is repeated as long as necessary to remove by-products and low molecular weight materials that are more soluble in the methanol than the desired polymer. In this manner, the undesirable monomeric impurities and oligomers are soluble in the solvent (such as methanol) depending upon temperature and thus are removed during each fractionation step.